Judge Wenche Elizabeth Arntzen said in Oslo that the new evaluation is necessary considering widespread criticism of the initial findings, which suggested Breivik should be sent to psychiatric care instead of prison.
Arntzen said two Norwegian psychiatrists -- Agnar Aspaas and Terje Toerrisen -- had been appointed for the new evaluation.
Breivik has confessed to a bomb and shooting spree on July 22 that killed 77 people.
He denies criminal guilt, saying he's a commander of a resistance movement aiming to overthrow European governments and replace them with "patriotic" regimes that will deport Muslim immigrants.
Investigators have found no sign of such a movement and say Breivik most likely plotted and carried out the attacks on his own.
The first court-ordered assessment found the 32-year-old Norwegian was psychotic during the attacks, which would make him mentally unfit to be convicted and imprisoned for the country's worst peacetime massacre.
That conclusion drew criticism from many outside experts who questioned whether someone who is suffering from a grave mental illness could carry out such a well-planned attack.
Asked what would happen if the new assessment conflicts with the first one, Arntzen said both reports would be considered by the court when the trial starts in April.